1. Field of the Invention:
The present invention relates to a high rigidity bearing used for the rotary shaft of a machine tool and a method for manufacturing the same.
2. Description of the Prior Art:
In a machining operation, an excessive load is apt to be applied to rotary shafts, particularly to a main spindle. Therefore, a high rigidity fluid bearing such as a hydrodynamic pressure effect bearing has been employed to maintain the rotary shaft accurately in its original axial position. In such a hydrostatic bearing the bearing clearance on a land portion which is formed on the bearing surface of a bearing bushing could not be formed with a small enough clearance which would be adequate for generation of sufficient hydrodynamic pressure because a hydrostatic pressure is generated in a large bearing clearance in comparison with the hydrodynamic pressure.
In a conventional method for manufacturing such a fluid bearing, a large radial force is applied by means of clamping bolts, circumferentially located around the bearing bushing, to the outer periphery of the bearing bushing. In this state, the bearing surface is formed to a true or proper circle on the bore of the bearing bushing. Accordingly, when the radial force is removed from the bearing bushing, a plurality of raised lands which have been held in an inwardly urged state by the clamping bolts during the machining operation are outwardly retracted from the hydrostatic pressure-generating land portion so as to form wedge-shape clearances between the rotary shaft and the raised lands. Thus, the hydrodynamic pressure is generated in the wedge-shape clearances when the rotary shaft is rotating.
This prior art manufacturing method did not, however, provide accurate wedge-shaped clearances to the hydrostatic bearing because it was difficult to accurately set the amount of thrust for the clamping bolts and it was also difficult to positively support the bearing bushing by the clamping bolts in a stable state during the machining operation of the bore of the bearing bushing. Furthermore, since the raised lands were arranged so as to be retracted away from the outer periphery of the rotary shaft beyond the surface of the hydrostatic pressure-generating land portion, the wedge-shape clearances were made wider than that of the hydrostatic pressure-generating land portion. As a result, the hydrostatic bearing which was manufactured by the above-mentioned method could not obtain a sufficient hydrodynamic pressure effect.